Bimolecular encounters: Acid-base and/or electron transfer in ruthenium (II) complexes. Experiment and theory

This work correlates experimental and theoretical studies of ruthenium diimines, [Ru(bpy)2L]2+ where L = bpy, or dpp. Experimental research concentrates on excited state acid-base and electron transfer properties. Although the driving force for reduction of Fe(III) and Ag(I) are essentially equivalent, data presented here show that the bimolecular encounter between bis(2,2'-bipyridine)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), [Ru(bpy) 2(dpp)]2+ in its dpp localized MLCT state, leads to coordination of Ag+, but electron transfer to Fe3+. Several redox reactions were performed between [Ru(bpy)2dpp]2+ and a strong oxidizing agents to analyze the spectroscopic signature of the oxidized form of the complex, [Ru(bpy)2dpp]3+. The experiments confirm electron transfer between excited state of bis(2,2'-bipyridine(2,3-bis(2pyridyl)pyrazine)ruthenium(II), [Ru(bpy)2dpp]2+ and iron(III) in aqueous and buffered solutions. Electron transfer has been confirmed by trapping Fe(II), one of the products of excited state electron transfer. The efficiency of excited electron transfer is influenced by solvent cage, where redox pair products form by competition between back reaction and diffusion process.;Two molecules of ruthenium complexes, [Ru(bpy)3]n+ and [Ru(bpy)2dpp]n+ , where n = 2+, 3+, have been investigated theoretically with a particular attention to their changes in electron density. Structural optimizations and energy calculations were performed by the Density Functional Theory (DFT) method in the Gaussian09 package including the effective core potentials (Los - Alamos ECP) for the ruthenium(II) ion. Electronic absorption spectra have been calculated and compared with experimental data. Computation of spectroscopic properties and electrochemical energies agree with experimental findings and provide a rationale for the spectroscopic properties of the oxidized form of [Ru(bpy)2dpp] 3+. The possibility of proton-coupled electron transfer (PCET) reaction was investigated by examining the energetics and pKa's of all possible forms of [Ru(bpy)2dpp]2+, which suggested that the quenching by iron(III) is an electron transfer process. Theoretical electron transfer rates between iron(III) and excited state of [Ru(bpy)2dpp] 2+ calculated with Marcus cross-relation are in good agreement with experimental findings.